Unexamined Published Japanese Patent Application No. H1-109158 discloses an antilock brake system (ABS) equipped with an auxiliary hydraulic pressure source device provided with an accumulator that stores high-pressure brake fluid. A hydraulic booster operating with this high-pressure brake fluid actuates a tandem master cylinder. A hydraulic brake pressure control valve is mounted between the tandem master cylinder and one wheel cylinder, while a similar hydraulic brake pressure control valve is interposed between the tandem master cylinder and another one of the wheel cylinders. When skid control is activated, the inlets of the hydraulic brake pressure control valves that are connected with the master cylinder are placed in communication with the working chamber of the hydraulic booster. Thus, brake fluid under pressure is sent from the working chamber of the hydraulic booster to the wheel cylinders.
Unexamined Published Japanese Patent Application No. S60-38243 discloses another antilock brake system (ABS) equipped with an auxiliary hydraulic pressure source device having an accumulator that stores high-pressure brake fluid. A hydraulic booster operating with this high-pressure brake fluid actuates a master cylinder and brake fluid under pressure is supplied from the master cylinder to the wheel cylinders of the nondriving wheels. Brake fluid under pressure is supplied from the working chamber of the hydraulic booster to the wheel cylinders of the drive wheels. The brake fluid under pressure is supplied to the wheel cylinders of the drive wheels from the working chamber of the hydraulic booster. A hydraulic brake pressure control valve is mounted between the master cylinder and the wheel cylinders of the nondriving wheels. A similar hydraulic brake pressure control valve is also interposed between the working chamber of the hydraulic booster and the wheel cylinders of the drive wheels.
The auxiliary hydraulic pressure source device described in the above-cited Unexamined Published Japanese Patent Application No. H1-109158 includes a high-pressure brake fluid output line that communicates with the accumulator, a reservoir which stores low-pressure brake fluid, a hydraulic pump which elevates the pressure of the low-pressure brake fluid in the reservoir and pumps the brake fluid to the high-pressure fluid output line, and a hydraulic pump driving device (e.g., an electric motor) that drives the hydraulic pump. A control device maintains the hydraulic pressure inside the accumulator by effecting on and off operation of the hydraulic pump in response to variations in the hydraulic pressure in the accumulator.
In a vehicle equipped with an antilock brake system having the auxiliary hydraulic pressure source device of the construction described above, if hard braking takes place on dry asphalt pavement, high-pressure brake fluid stored in the accumulator is consumed at a very high rate at the initial stage of the braking action. The result is that the hydraulic pressure in the accumulator and the high-pressure brake fluid output line through which high-pressure brake fluid is fed to the hydraulic booster drops very rapidly down to a very small level.
Where the accumulator, the hydraulic pump, and the hydraulic pump-driving device are made to be smaller in size so that the auxiliary hydraulic pressure source device is more conveniently mounted in a vehicle, the hydraulic pressure in the high-pressure brake fluid output line drops very rapidly to a very small level. As a result, the pressure becomes lower than the hydraulic brake pressure value at which the wheels are locked. The hydraulic pressure value in the high-pressure brake fluid output line at which the operation of the hydraulic pump is started is normally set higher than the hydraulic brake pressure at which the wheels are locked on a dry asphalt pavement. The hydraulic pump is driven in response to large drops in the hydraulic pressure inside the high-pressure brake fluid output line. The pump pressurizes the low-pressure brake fluid in the reservoir and begins pumping this fluid to the high-pressure brake fluid output line. Because the high-pressure brake fluid output line is in communication with the accumulator, the brake fluid supplied from the hydraulic pump to the high-pressure brake fluid output line flows into the accumulator from the beginning. It thus takes a relatively long time before the hydraulic pressure in the high-pressure brake fluid output line reaches the hydraulic brake pressure at which the wheels are locked. If skid control is started before the hydraulic pressure inside the high-pressure brake fluid output line reaches the hydraulic brake pressure at which the wheels are locked, the pressure is again increased but with a delay after the brake fluid pressure inside the wheel cylinders is decreased. Consequently, a large deceleration cannot be obtained. Hence, the performance of the brakes deteriorates. Accordingly, with the above-described auxiliary hydraulic pressure source device, the capacity of the accumulator is set to be large in an attempt to increase without delay the hydraulic brake pressure inside the wheel cylinders, even if hard braking takes place on a dry asphalt pavement, and if skid control is immediately started. As a result, the above-described auxiliary hydraulic pressure source device is rather large in size and cannot be conveniently mounted in a vehicle.
In light of the foregoing, a need exists for an auxiliary hydraulic pressure source device that is free of the foregoing problems, drawbacks and disadvantages.
It would thus be desirable to provide an auxiliary hydraulic pressure source device that is able to increase without delay the hydraulic brake pressure inside the wheel cylinders, even if hard braking takes place on a dry asphalt pavement and if skid control is immediately started.
It would also be desirable to provide an auxiliary hydraulic pressure source device that is able to achieve such a result without excessively increasing the size of the device and while still allowing the auxiliary hydraulic pressure source device to be conveniently mounted within a vehicle.